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Full wave loop, high SWR

Crawdad

Down in the mud invasive species
Nov 11, 2016
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Am experimenting with a Full Wave, Vertically Oriented, Rectangular Loop antenna on a condo balcony. Wire is #12 AWG THHN cut for 27.395 Mhz. Short sides (4'-6") of rectangle are vertical, long sides (13'-6") are horizontal. Antenna is fed in the center of one of the short vertical sides with a quarter wave length of 75 ohm (RG-6) coax, coupled to RG-8X into the shack. Feedpoint is approximately 17' above ground. No analyzer so no real idea of actual impedance of the loop. Cannot get SWR below 3.5 @ center frequency. Previous antenna was an Inverted V dipole that matched with no problem. Any ideas/help greatly appreciated.
Thanks
 

Am experimenting with a Full Wave, Vertically Oriented, Rectangular Loop antenna on a condo balcony. Wire is #12 AWG THHN cut for 27.395 Mhz. Short sides (4'-6") of rectangle are vertical, long sides (13'-6") are horizontal. Antenna is fed in the center of one of the short vertical sides with a quarter wave length of 75 ohm (RG-6) coax, coupled to RG-8X into the shack. Feedpoint is approximately 17' above ground. No analyzer so no real idea of actual impedance of the loop. Cannot get SWR below 3.5 @ center frequency. Previous antenna was an Inverted V dipole that matched with no problem. Any ideas/help greatly appreciated.
Thanks

Full Wave antennas are a real pain from all I have ever read about them on any band so most people avoid them & go with a 1/2 wave or a 1/4 wave & they work very well with few issues.The ARRL Antenna Handbook is a great reference if you don't have one.

SIX-SHOOTER
 
Full wavelength loops have an impedance closer to 100 ohms, so the 1/4 wavelength of 75 ohm coax should have gotten close. However, these coaxial impedance transformers can be picky when it comes to actual length.

Did you factor in the velocity of said RG-6 coax? As it recall it has a velocity factor of close to 80%? You will want to double check my number on this, but it should be listed on the side of the coax. I am asking because it wasn't mentioned, and is a common mistake people make.


The DB
 
Full wavelength loops have an impedance closer to 100 ohms, so the 1/4 wavelength of 75 ohm coax should have gotten close. However, these coaxial impedance transformers can be picky when it comes to actual length.

Did you factor in the velocity of said RG-6 coax? As it recall it has a velocity factor of close to 80%? You will want to double check my number on this, but it should be listed on the side of the coax. I am asking because it wasn't mentioned, and is a common mistake people make.


The DB
Thanks DB,

Yes I should have mentioned that. That particular RG-6 I used was a poly foam with a V/F of 0.82 so I cut it to the proper length. I had also tried a piece of RG-59 with a V/F of 0.66 but the SWR was even higher. I don't have much experience with those impedance transformers so I'm not sure if they might need to be longer or shorter than what the formula says.
 
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Could be several things. Loops are the only wire antenna I will bother using on Hf .... but these are bigger loops and are horizontal loops strung out to trees. I am not familiar with using the 75ohm cable section as some sort of matching stub. Bigger loops commonly use a 4:1 coaxial balun at the feedpoint (if you're running coax out to the antenna). 450 ohm ladderline is the other typical option .... which comes back to the shack and attaches to the Wire Antenna ports on a antenna tuner.
It could be working but with this configuration/location you might be seeing reflection interference from being right next to your building. At any rate, you may need to use an Antenna Tuner inline to make the rest of the match.
Also, a Vertical Loop like this is going to be Directional front & back and not omni. All Vertical Loops are directional like this. So half the output is coming right back towards the building and may be causing reflection issues.
Good luck. Let us know how it turns out.
 
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Today I resized the rectangle closer to a 2 to 1 ratio, relocated the feedpoint to a long side and changed to a shorter piece of RG-6 wound as a RF choke. The loop then tuned down to 1.6 on my target frequency. Made some local and DX contacts. Thanks all for your responses.
 

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